Alloy steel for low temperature vacuum carburizing

ABSTRACT

The present invention relates to an alloy steel for low temperature vacuum carburizing, and more particularly, to an alloy steel for low temperature vacuum carburizing, wherein, where the thermal processing of carburizing and quenching is performed at 810° C. or so of an available minimum carburizing temperature of a conventional vacuum carburizing furnace, it is able to securing an adequate ferrite phase (α) to improve the thermal distortion according to the thermal processing of an annulus gear, and to satisfying shape restrictions such as a roundness or a cylindricity of the annulus gear to be manufactured. An alloy steel for a low temperature vacuum carburizing according to the present invention, the alloy steel is composed of a chief element of Fe, wherein the alloy steel is formed so that dissolved oxygen (DO) is 10 ppm or less in an alloy system which comprises 0.17˜0.24 weight percent of C, 0.8˜1.2 weight percent of Cr, 0.4˜0.8 weight percent of Mn, 0.80˜1.20 weight percent of Si, 0.020 weight percent or less of P, 0.020 weight percent or less of S, 0.015˜0.045 weight percent of V, and the remaining weight percent of Fe.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims under 35 U.S.C. §119(a) to the benefit of Korean Patent Application Number 10˜2009˜0117993, filed on 1, Dec., 2009, the entire contents of which is incorporated herein for all purposes by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, generally, to an alloy steel for low temperature vacuum carburizing, and more particularly, to an alloy steel for low temperature vacuum carburizing, wherein, when the thermal processing of carburizing and quenching is performed at 810° C. or so of an available minimum carburizing temperature of a conventional vacuum carburizing furnace, it is able to securing an adequate ferrite phase (α) to suitably improve the thermal distortion according to the thermal processing of an annulus gear, and to suitably satisfy shape restrictions of the annulus gear to be manufactured, such as, but not limited to, roundness or cylindricity.

2. Description of Prior Art

In general, for gear-like components of a vehicle transmission, a thermal processing of carburizing and quenching is typically performed. When the thermal processing of carburizing and quenching is performed, distortion of the components may occur. In certain cases where the distortion is excessive, it may cause problems with the assembly of the components, or unusual noise may occur.

FIG. 1 shows an exemplary general annulus gear.

As shown in FIG. 1, an annulus gear is a cylindrical component, wherein the annulus gear is a main component of a planetary gear train applicable to an important power transmission system of an automatic transmission. Preferably, the annulus gear is an internal gear whose teeth are suitably processed through a broaching processing. Since the annulus gear has a large diameter and a thin thickness, it is very sensitive to thermal distortion. Further, while teeth surface processing of a general external gear is easily performed after the thermal processing such as grinding or the like, the annulus gear is one of the components whose processing is difficult after the thermal processing.

There are various thermal processing methods of the annulus gear that encompass the thermal distortion.

For example, in order to minimize the thermal distortion of the annulus gear, a series of methods of gas carburizing, furnace cooling, high frequency heating and jig quenching have been used. By performing the final jig quenching processing, that is, by applying high frequency heating, jig assembling, and then quenching, it is possible to satisfy the shape restrictions such as the roundness, the cylindricity, and the like.

However, the above-described method involves long and complex processing. Further, since the method is individually performed by components, the operation efficiency may suitably decrease. Further, when the jig is cooled, a shape of gear teeth may suitably deform unevenly by a localized uneven cooling, and accordingly, unusual noise may occur.

Recently, vacuum carburizing and gas quenching methods have been applied, which have considerable productivity and uniform cooling. It has been possible to improve the noise which is caused by uneven deformation; however, aspects of the shape such as roundness, cylindricity, or the like may still be improved.

Accordingly, new compositions and methods for low temperature vacuum carburizing are needed.

The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention provides an alloy steel for a low temperature vacuum carburizing, wherein, at 810° C. or so of an available minimum carburizing temperature of a conventional vacuum carburizing furnace, it is able to reach an adequate ferrite phase (α).

In preferred embodiments, the present invention provides an alloy steel for a low temperature vacuum carburizing, the alloy steel preferably being composed of a chief element of Fe, wherein the alloy steel is formed so that dissolved oxygen (DO) is 10 ppm or less in an alloy system which comprises 0.17˜0.24 weight percent of C, 0.8˜1.2 weight percent of Cr, 0.4˜0.8 weight percent of Mn, 0.80˜1.20 weight percent of Si, 0.020 weight percent or less of P, 0.020 weight percent or less of S, 0.015˜0.045 weight percent of V, and the remaining weight percent of Fe.

Preferably, the alloy steel for a low temperature vacuum carburizing of the present invention, in certain preferred embodiments, where the thermal processing of carburizing and quenching is performed at 810° C. or so of an available minimum carburizing temperature of a conventional vacuum carburizing furnace, reaches an adequate ferrite phase (α) to suitably improve the thermal distortion according to the thermal processing of the annulus gear, and to suitably satisfies shape restrictions such as a roundness or a cylindricity of the annulus gear to be manufactured.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum).

As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered.

The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description, which together serve to explain by way of example the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated by the accompanying drawings which are given hereinafter by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a perspective view for illustrating a general annulus gear of the prior art.

FIG. 2 is a view for explaining a concept of a low temperature vacuum carburizing, and illustrates a state of Fe—C (iron-carbon).

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described herein, the present invention features an alloy steel for a low temperature vacuum carburizing. Preferably, the alloy steel is comprised mainly of Fe.

In preferred aspects, the present invention features an alloy steel for a low temperature vacuum carburizing, the alloy steel being composed of a chief element of Fe, wherein the alloy steel is formed so that dissolved oxygen (DO) is 10 ppm or less in an alloy system which comprises 0.17˜0.24 weight percent of C, 0.8-1.2 weight percent of Cr, 0.4-0.8 weight percent of Mn, 0.80-1.20 weight percent of Si, 0.020 weight percent or less of P, 0.020 weight percent or less of S, 0.015-0.045 weight percent of V, and the remaining weight percent of Fe.

An alloy-steel for a low temperature vacuum carburizing according to preferred embodiments of the present invention will be explained in detail.

A the low temperature carburizing method has been studied as a substitute for the conventional jig quenching method in the aspect of improving the shape such as roundness, cylindricity, or the like, by applying the vacuum carburizing method. FIG. 2 illustrates an example of the low temperature vacuum carburizing method, and illustrates a state of Fe—C (iron-carbon).

Referring to FIG. 2, the low temperature carburizing method has carried out the carburizing processing at a temperature area between dual phases A1˜A3 in which an austenite phase (γ) and a ferrite phase (α) exist together. Accordingly, the low temperature carburizing method improves the thermal deformation, since a defined amount of the ferrite phase (α) is suitably maintained without a phase change at a temperature elevation process and carburizing process of products, and it is also maintained in the quenching processing. However, in case of the vacuum carburizing furnace which is being mass-produced now, since it is possible to mass-produce in carburizing over a temperature of 800˜810° C. according to the characteristics of the equipment. Therefore, in case where a carburizing material which is commonly used may be carburized and quenched in an available carburizing temperature of 810° C., it may not sufficiently improve the thermal deformation since the ferrite phase (α) is secured not to enough.

As shown in the following Table 1, the alloy steel for the low temperature vacuum carburizing according to preferred embodiments of the present invention is composed of Fe, and preferably a chief element of Fe, wherein the alloy steel is formed so that dissolved oxygen (DO) is 10 ppm or less for a cleanliness level of the alloy (in order to decrease a content of impurities) in an alloy system which comprises 0.17˜0.24 weight percent of C, 0.8˜1.2 weight percent of Cr, 0.4˜0.8 weight percent of Mn, 0.80˜1.20 weight percent of Si, 0.020 weight percent or less of P, 0.020 weight percent or less of S, 0.015˜0.045 weight percent of V, and the remaining weight percent of Fe.

TABLE 1 Alloy compositions of the steel of the present invention and the conventional comparative steel (Unit of the content: weight percent) O Classification C Cr Mn Si P S V (ppm) Fe Steel of the 0.17~0.24 0.8~1.2 0.4~0.8 0.8~1.2 below below 0.015~0.045 below rest invention 0.02 0.02 10 Comparative 0.17~0.23 0.85~1.25 0.55~0.9  0.15~0.35 below below — below rest Steel 0.03 0.03 25 (SCr420H)

In preferred exemplary embodiments, the alloy steel for the low temperature vacuum carburizing according to the present invention may be mainly designed as follows.

The alloy steel of the present invention is designed to secure an adequate ferrite phase (α) at 810° or so of an available minimum carburizing temperature of a conventional vacuum carburizing furnace. Preferably, since the alloy steel is designed so that a temperature A3 of the alloy steel of the present invention referring to FIG. 2, may secure over 850° C., the 30˜50% of the ferrite phase (α) may also exist in a deep core of the alloy steel through the carburizing and quenching of 810° C.˜830° C.

The alloy steel according to the present invention will be explained in detail as follows.

In certain preferred embodiments, carbon (C) may comprise a content of 0.17˜0.24 weight percent so as to have an adequate hardness of the core after the high frequency carburizing. In certain preferred embodiments where the content is less than the normal amount, the ferrite phase (α) is contained too much so that the hardness of the core becomes lower, and accordingly, the hardness of the alloy steel becomes lower. In certain examples where the content is more than the normal amount, the ferrite phase (α) is contained too little, and accordingly, it is deficient in improving the thermal deformation.

Preferably, a chromium (Cr) which is an important element to enhance a fatigue strength of a gear steel may comprise a content of 0.8˜1.2 weight percent. Preferably, since the chromium may reduce the temperature of A3 as well as the temperature of A1 referring to FIG. 2, the amount of the chromium is suitably restricted. In particular embodiments, the chromium is added more than 0.8 weight percent in order to suitably reduce of the temperature of A1 and to suitably secure the fatigue strength, and it is restricted less than 1.2 weight percent because it may not secure the adequate ferrite phase (α) according to the temperature of A3 which is reduced.

According to further preferred embodiments, manganese (Mn) may comprise a content of 0.4˜0.8 weight percent to be decreased in comparison with the conventional carburizing steel.

Although the manganese (Mn) may suitably increase the strength of the alloy steel, it is an element which may be a cause of the uneven thermal deformation by suitably inducing the segregation in steelmaking. In particular preferred embodiments, the manganese (Mn) was added more than 0.4 weight percent to decrease the temperature of A1, and the manganese (Mn) was restricted less than 0.8 weight percent to reduce the segregation.

Preferably, in order to secure the adequate amount of the ferrite phase (α) by suitably increasing the temperature of A3, the content of silicon (Si) was increased and the amount of vanadium (V) was adequately added.

In further preferred embodiments, the silicon (Si) preferably comprises 0.80˜4.20 weight percent so as to suitably increase the temperature of A3 and to suitably increase a characteristic of the contact fatigue (pitting resistance) as well. In certain exemplary embodiments, for example where the content of the silicon (Si) is less than the normal amount, since the temperature of A3 is not increased enough, it is difficult to secure an adequate amount of the ferrite phase (α). Further, in case where the content of the silicon (Si) is more than 1.2 weight percent, a known solid-solution strengthening is too much so as to reduce exceedingly the formability, and accordingly, the forging process and the manufacturing process may be suitably difficult. Preferably, the alloy steel of the present invention is suitable to the vacuum carburizing processing since the amount of the silicon is very much in comparison with the conventional steel. In certain preferred embodiments where the carburizing is suitably performed in the conventional gas carburizing furnace, surface oxide layers may be formed by the oxidizing atmosphere, and accordingly, the durability of the alloy steel may be suitably reduced.

According to other further embodiments, the vanadium (V) preferably comprises 0.015˜0.045 weight percent so as to suitably increase the temperature of A3 and to further maximize a grain refinement. Preferably, since the vanadium (V) is an element for a fine precipitation strengthening, in further preferred embodiments, the amount of the vanadium (V) was restricted to be more than 0.015 weight percent to suitably improve the hardness of the core according the fine precipitation strengthening, and to increase the temperature of A3 as well. Further, the amount of the vanadium (V) was restricted to be less than 0.045 weight percent since the price of the vanadium (V) is very expensive.

According to further exemplary embodiments, dissolved oxygen (DO) was formed to be 10 ppm or less. Since a hardness of the core in the alloy-steel of the present invention is comparatively low in comparison with those of the conventional alloy steel for carburizing, the requisite characteristics of a surface layer (a carburizing layer) are considered, and accordingly, the dissolved oxygen was suitably restricted to be 10 ppm or less. Preferably, if the dissolved oxygen is more than 10 ppm, it may affect the performance for surface contact fatigue.

EXAMPLES

In the following Table 2, the chemical compositions and the important phase deformation temperature of the alloy steel of the present invention and the comparative steel are illustrated.

TABLE 2 Phase deformation Available temperature carburizing Chemical component (weight percent) (° C.) Temperature Classification C Cr Mn Si P S V O (ppm) Fe Al A3 (° C.) Steel of 0.22 1.0 0.6 1.1 0.015 0.01 0.04 8 rest 761 852 770~840 invention Comparative 0.2 1.1 0.8 0.2 0.015 0.015 — 17 rest 743 803 755~790 steel (SCr42OH)

In case of the comparative steel as the conventional steel, when it is carburized at 810° or so of an available minimum carburizing temperature of a conventional vacuum carburizing furnace, it may predict that the amount of the ferrite phase (α) is not in existence since it is carburized at a single phase area of the austenite phase (γ) over the temperature of A3. In case of the alloy steel of the present invention, t may secure the adequate amount of the ferrite phase (α) and thus improve the thermal deformation.

The comparative examples are illustrated in the following Table 3, wherein they are carburized at the actual temperature of 810°, respectively.

TABLE 3 Physical properties for thermal processing Hardness (Hv) the a Average size after (surface/ amount of thermal processing Thermal processing deep the core (μm) Classification condition

(%) Roundness Cylindricity Remarks Embodiment Vacuum carburizing 750/340 about 35 38 42 Low (steel of the (810□) temperature invention) □ High pressure carburizing gas cooling of steel of (130 bar, Helium) the invention Comparative 1 760/390 about 2  53 72 Low example temperature (conventional carburizing steel) of the conventional steel 2 Gas carburizing 750/400 0 36 40 Jig (920□) quenching □ Furnace cooling □ High frequency heating □ Jig quenching (Oil cooling) Used test component: Rear annulus gear for 6-speed automatic transmission (the inside diameter of gear IBD: 122.45 mm (Gear of FIG. 1) Average size after thermal processing is a mean value of 20 test products

indicates data missing or illegible when filed

In certain examples where the alloy steel of the present invention was suitably performed in low temperature vacuum carburizing at the temperature of 810°, it was shown that the thermal deformation was considerably improved after the thermal processing, in comparison with the conventional steel which is performed in the same thermal processing. In case of the conventional carburizing material, the ferrite phase (α) was not adequately secured at the deep core in carburizing at this temperature. In case of the alloy steel of the present invention, the ferrite phase (α) was adequately secured.

Further, the result was almost equal in comparison with the conventional jig quenching method.

As the above-described, in case where the thermal processing of carburizing and quenching is performed at 810° or so of an available minimum carburizing temperature of a conventional vacuum carburizing furnace, the alloy steel for the low temperature vacuum carburizing according to the present invention is able to secure an adequate ferrite phase (α) to improve the thermal distortion according to the thermal processing of an annulus gear, and to satisfy shape restrictions such as the roundness or the cylindricity of the annulus gear to be manufactured. 

1. An alloy steel for a low temperature vacuum carburizing, the alloy steel being composed of a chief element of Fe, wherein the alloy steel is formed so that dissolved oxygen (DO) is 10 ppm or less in an alloy system which comprises 0.17˜0.24 weight percent of C, 0.8-1.2 weight percent of Cr, 0.4-0.8 weight percent of Mn, 0.80-1.20 weight percent of Si, 0.020 weight percent or less of P, 0.020 weight percent or less of S, 0.015-0.045 weight percent of V, and the remaining weight percent of Fe.
 2. An alloy steel for low temperature vacuum carburizing, the alloy steel comprising 0.17˜0.24 weight percent of C, 0.8-1.2 weight percent of Cr, 0.4-0.8 weight percent of Mn, 0.80-1.20 weight percent of Si, 0.020 weight percent or less of P, 0.020 weight percent or less of S, 0.015-0.045 weight percent of V, and the remaining weight percent of Fe.
 3. The alloy steel for low temperature vacuum carburizing of claim 2, wherein the alloy steel is formed so that dissolved oxygen (DO) is 10 ppm or less.
 4. An alloy steel for low temperature vacuum carburizing, the alloy steel comprising Fe, wherein the alloy steel is formed so that dissolved oxygen (DO) is 10 ppm or less.
 5. The alloy steel for low temperature vacuum carburizing of claim 4, wherein the alloy comprises 0.17˜0.24 weight percent of C, 0.8-1.2 weight percent of Cr, 0.4-0.8 weight percent of Mn, 0.80-1.20 weight percent of Si, 0.020 weight percent or less of P, 0.020 weight percent or less of S, 0.015-0.045 weight percent of V, and the remaining weight percent of Fe. 